Electrothermal fluid displacement actuators that convert electrical energy into thermal energy and, in turn, employ the thermal energy to expand a thermally expandable fluid medium and produce linear motion are known. Examples of such actuators are described in commonly assigned U.S. Pat. Nos. 4,029,941 and 4,070,946, the disclosures of which are incorporated herein by reference. Additional examples of electrothermal actuators are disclosed in U.S. Pat. Nos. 4,070,859, 4,079,589, 4,104,507, 4,759,189, and 4,887,429, all commonly assigned.
In the electrothermal actuators described in the cited patents, a working fluid is contained within a boiler chamber in the actuator. Preferably, that fluid is a liquid at room temperature but, upon heating to a sufficient temperature, changes from a liquid phase to a gas phase. The phase transition results in an increasing pressure within the boiler chamber. The expanding working fluid presses against a diaphragm and displaces and/or expands the diaphragm to slide a piston within the actuator and a pin attached to the piston outwardly from the actuator. The out-stroking motion of the piston may be employed to actuate an external device. A return spring mounted within the actuator biases the piston to retract the pin when the pressure within the boiler chamber decreases. As long as the pressure of the working fluid is sufficient to overcome the biasing force of the return spring, the pin remains extended from the actuator. When the working fluid cools and returns to a liquid phase, the pressure on the diaphragm is reduced, the diaphragm contracts and/or retracts, and the piston and pin retract under the influence of the biasing force applied by the return spring.
In order to heat and produce a phase change in the working fluid, an electrically-driven heater is disposed within the boiler chamber. The heater may be a simple resistance heater that produces heat in response to a current flow through the heater. The heater may be a positive temperature coefficient (PTC) heating element that significantly increases in resistance once a particular temperature is reached. A PTC heater inherently limits the magnitude of the steady state current that flows in response to a particular voltage applied to the heater. To improve actuator response time without excessive current flows, a resistance heater may be connected in series with a PTC heating element.
In all of the thermal actuators described in the cited patents, a sliding piston extends and retracts a pin in response to an electrical signal applied to the heater. In many applications, it is desirable to provide a rotary motion rather than the linear motion produced by the known actuators.